This invention relates to improved two phase anaerobic digestion having separated hydrolysis and biogasification reactors which convert biomass to desired methane product gas with high efficiency.
Anaerobic digestion has been known to stabilize sludge and other predominantly organic materials, and usable product gas, of varying composition, has been obtained from such anaerobic digestion processes. The organic feed mixture which provides the substrate for anaerobic biodegradation can comprise a wide variety of organic carbon sources, ranging from raw sewage sludge to municipal refuse, or biomass material such as plants and crop wastes. The process of anaerobic digestion degrades any of these organic carbonaceous materials, under appropriate operating conditions, to product gas which contains the desirable methane gas.
Anaerobic digestion uses a consortium of natural bacteria to degrade and then convert an organic substrate into a mixture of carbon dioxide and methane. The existing anaerobic digestion systems for organic substrate digestion can be separated into two major types, one phase systems and two phase systems. Existing one phase systems include the batch digester, completely mixed digester and the plug flow digester. These one phase systems, in which the organic substrate and the microorganisms are housed together are easy to operate and of low cost. Completely mixed digesters and plug flow digesters require continuous handling of feedstock and do not operate in batch mode. Further, the biogas produced in one phase systems consists primarily of carbon dioxide in the early stages of digestion. The high carbon dioxide content of the biogas is attributable to the slow growth of the methanogenic microorganisms and their inhibition by high concentrations of volatile fatty acids (VFAs). In order to reduce the inhibition of the microorganisms by the VFAs, the two phase digester has been introduced.
Separated two phase anaerobic digestion systems have been found to enhance the conversion efficiency, such as described in Pohland and Ghosh, Biotechnol. and Bio-eng. Symp. No. 2, 85-106 (1971), John Wiley and Sons, Inc. and by the same authors in Environmental Letters, 1: 255-266 (1971). A typical two phase anaerobic digester system comprises an acid phase digester and a biogasification reactor. The acid phase digester is usually designed as a solid-bed batch reactor where solid waste is housed and leached soluble compounds are collected. In the acid first phase, the microbial population and operating conditions are selected to promote the conversion of organic carbonaceous materials to carbonaceous materials of lower molecular weight, primarily volatile fatty acids. The liquid and solid effluent from the acid phase is conveyed to a biogasification second phase, where methanogenic organisms convert the volatile fatty acids to product gas that is composed primarily of methane and carbon dioxide. Product gas is removed from the biogasification reactor and processed, or scrubbed, to separate the methane component that is drawn off as pipeline gas.
Two phase anaerobic digestion has been carried out in a single reactor as taught, for example, by U.S. Pat. No. 4,735,724 which teaches a non-mixed vertical tower anaerobic digester and anaerobic digestion process which provides passive concentration of biodegradable feed solids and microorganisms in an upper portion of a continuous digester volume and effluent withdrawal from the middle to the bottom portion of the digester, resulting in increased solids retention times, reduced hydraulic retention times and enhanced bioconversion efficiency.
U.S. Pat. No. 4,022,665 discloses certain specific operating conditions for a two phase anaerobic digestion process, such as feed rates and detention times, which promote efficient conversion of organic materials. Additionally, the ""665 patent discloses two separated biogasification reactors, a biogasification reactor I operated in series with a biogasification reactor II. The biogasification reactor II receives effluent fluid and/or effluent gas from biogasification reactor I. A somewhat similar process is disclosed in U.S. Pat. No. 4,696,746 which teaches a process for two phase anaerobic digestion with two discrete biogasification reactors operated in parallel.
U.S. Pat. No. 3,383,309 teaches that the rate and efficiency of the anaerobic digestion process, particularly in the methane forming phase, are increased when hydrogen gas is introduced into the digester sludge. According to the ""309 patent, hydrogen gas is introduced into both the acid forming and the methane forming phases, to increase the availability of energy rich xe2x80x9chyper-sludge.xe2x80x9d All improvements disclosed in U.S. Pat. Nos. 4,022,665, 4,696,746 3 and 383,309 can be adapted for use according to the improved process of the present invention and the teachings of that patent are incorporated herein by reference.
French Patent No. 78 34240 describes an apparatus for biogasification which is known in the art as an upflow sludge blanket reactor. This apparatus utilizes a two-stage digestion apparatus. The apparatus is designed for and uses continuous recirculation between the reactors of the two stages. Continuous recirculation requires a relatively complex apparatus including filters, pumps and manifolded inlets to disperse the recirculated liquid stream and to avoid its xe2x80x9cshort circuiting directly to the outlet of the reactor into which it was just circulated. Additionally, the continuous recirculation requires two pumps that must operate continuously. In contrast, the present invention utilizes intermittent recirculation.
The sequential batch anaerobic composting (SEBAC) reactor is a relatively new digestion system. See, Chynoweth et al., Appl. Biochem. Biotech. 28: 421-32 (1991). The SEBAC system consists of three reactors. Each reactor operates as a single phase batch digester. The three reactors are interconnected and operated on a different digestion schedule, the first being newly started, the second running in the middle of a digestion and the third running toward the end of a digestion. When new feedstock is loaded into the first reactor, the liquid from the third reactor is transferred to the first reactor to inoculate the feedstock and speed-up the digestion process.
A broad range of organic substrates are appropriate feedstocks for biogasification reactors. An exemplary feedstock is agricultural waste. Agricultural waste consists mostly of carbonaceous organic materials and it presents a particularly attractive renewable source of raw material for the generation of methane. The use of agricultural waste for this purpose serves a dual purpose, it produces a useful product and reduces the volume of agricultural waste which must be disposed of Many different types of agricultural waste can be digested utilizing a two phase anaerobic digestion scheme. The waste from the production of rice provides a salient example.
In California, for examples large quantities of rice straw are produced each year as by-products of rice production. In the Sacramento Valley alone, 1,452,000 tons of rice straw were produced in the crop year of 1994-1995 (CARB-CDFA, Progress report on the phase down of rice straw burning in the Sacramento Valley Air Basin, Report To The Legislature, California Air Resources Board and California Department of Food and Agriculture (1995)). Due to lack of feasible conversion technologies, however, utilization of these materials for energy production has not become practical for the agricultural sectors.
Current methods for disposal of these agricultural residue materials have caused widespread public concerns with regard to their environmental impact. In the case of rice and wheat straw disposal, for example, open field burning is considered as a practice causing serious air pollution problems, because of the emissions of smoke and other air pollutants, such as gases, particles and aerosols.
Current California legislation (the Connelly-Areias-Chandler Rice Straw Burning Reduction Act of 1991) mandates the rice growers to phase down burning of rice straw, requiring a reduction in rice straw acreage burning to no more than 25% of the planted acreage or 125,000 acres in the Sacramento Valley by the year 2000, whichever is less. As a result, in 1994-95, about 59% of the rice straw was burned and 38.4% was disposed of in the fields by soil incorporation. Off-farm disposal of rice straw as livestock feed and materials for environmental mitigation and erosion control counted for only 0.6%. Rice growers are under extreme pressure to find alternative environmentally friendly methods for straw disposal and/or utilization. If no other practical straw disposal alternatives are developed to compensate for the burning phasedown, rice farmers will be forced to incorporate an estimated 72.9% of the straw production by the year 2000 to comply with the statutory rice straw burning phasedown requirements. However, available research and experience suggest that incorporation rates this high could potentially cause reduction in crop yield and increase of foliar disease and possible development of adverse soil conditions.
Rice straw is offered as a single relevant example. The disposal of other solid wastes presents similar problems and new economical technologies for solid waste disposal and/or utilization must be developed. Thus, a method for disposing of agricultural and other wastes which utilized an apparatus of simple design, required little expenditure of energy to operate and which produced methane as it reduced the volume of disposable solids would represent a significant advance. Quite surprisingly the present invention provides such methods and devices.
Anaerobic digestion of solid waste, particularly agricultural waste, is a promising technique for both generating energy and reducing the volume of waste which must be disposed of. The energy generated can be significant. For example, the energy content of a pound of rice straw is about 6,500 Btu (British Thermal Units), and the energy stored in the straw by growing crop each year in the Sacramento Valley is 1.95xc3x971012 Btu. Thus, it is realistic to consider agricultural waste as a renewable resource for energy generation.
Anaerobic digestion is an enhanced biodegradation process that offers a promising alternative approach for helping solve problems caused by agricultural waste such as the imminent rice straw disposal problems in concentrated rice production regions such as California. Anaerobic digestion uses a consortium of natural bacteria to degrade and then convert a large portion of solid waste into biogas, which is a mixture of methane and carbon dioxide. If captured, biogas can be utilized as a clean fuel for heat and power generation.
Anaerobic phase digestion (APS) is a new type of two phase system. The system employs at least one hydrolysis reactor and a biogasification reactor. In the APS digester system, carbon compounds in the organic substrates are liquefied into VFAs in the hydrolysis reactor. The soluble VFAs produced are transferred to the biogasification reactor at a controlled rate. This allows the maintenance of a stable pH level in the biogasification reactor so that the optimum growth rate of methanogenic bacteria can be achieved. In a first aspect, the present invention provides a process for methane production by two-phase anaerobic digestion of organic material. The process comprises incubating a first mixture having a solid organic component and an aqueous liquid component, under anaerobic conditions, in a hydrolysis digester having an upper portion and a lower portion and containing a hydrolysis means therein. After a first period of incubation, a portion of the liquid component of the first mixture residing in the lower portion of the hydrolysis digester is transferred to a methane phase digester having an upper portion and a lower portion and a methanogenesis means therein. In the methane phase digester, the first mixture is combined with the methanogenesis means to form a second mixture. The second mixture is incubated for a second period of time, generating methane. The second mixture is intermittently agitated, then allowed to remain still for a third period of time. After the third period of time, a portion of the second mixture residing in the upper portion of the methane phase digester is transferred to the hydrolysis phase digester.
The APS-digester system of the invention has innovative design features that allow it to handle the solid organic substrates effectively. The hydrolysis reactor is operated in a batch or semi-batch mode to ease the handling of solid materials, and the biogasification reactor operated continuously to maintain active bacterial culture in the system and to produce biogas at a relatively constant level. The device used in the system of the invention is of simple design and is economical to construct and operate.
In a second aspect, the present invention provides an anaerobic phased solids digester system for methane production. The system comprises a hydrolysis reactor which is separated into upper and lower portions by a perforated support means. The upper portion of the hydrolysis reactor has a hydrolysis reactor liquid inlet and the lower portion has a hydrolysis reactor liquid outlet. The device further comprises a biogasification reactor. The biogasification reactor has a biogasification reactor gas outlet and, optionally, an agitating means. Similar to the hydrolysis reactor, the biogasification reactor has an upper portion and a lower portion. The upper portion has a biogasification reactor liquid outlet and the lower portion has a biogasification liquid inlet.
The hydrolysis reactor and the biogasification reactor are connected via a series of conduits through which liquid from one reactor can be transferred to another reactor. Thus, the device also comprises a first conduit connecting the hydrolysis reactor outlet to the biogasification inlet and a second conduit connecting the biogasification reactor outlet with the hydrolysis reactor inlet.
Other features, objects and advantages of the present invention and its preferred embodiments will become apparent from the detailed description that follows.